1. Field of Invention
The invention relates generally to the field of oil and gas production. More specifically, the present invention relates to an initiation transfer between perforating guns in a perforating string.
2. Description of Prior Art
Perforating systems are used for the purpose, among others, of making hydraulic communication passages, called perforations, in wellbores drilled through earth formations so that predetermined zones of the earth formations can be hydraulically connected to the wellbore. Perforations are needed because wellbores are typically completed by coaxially inserting a pipe or casing into the wellbore. The casing is retained in the wellbore by pumping cement into the annular space between the wellbore and the casing. The cemented casing is provided in the wellbore for the specific purpose of hydraulically isolating from each other the various earth formations penetrated by the wellbore.
Perforating systems typically comprise one or more perforating guns strung together, these strings of guns can sometimes surpass a thousand feet of perforating length. In FIG. 1 a prior art perforating system 11 is shown having a perforating gun string 4 with perforating guns 6. The gun string 4 is shown disposed within a wellbore 1 on a wireline 5. The perforating guns 6 in the gun string 4 are usually coupled together by connector subs 13. A service truck 7 on the surface 9 generally accompanies perforating systems 11 for handling the upper end of the wireline 5. The wireline 5 typically is used for raising and lowering the gun string 4, as well as a communication means and control signal path between the truck 7 and the perforating gun 6. The wireline 5 is generally threaded through pulleys 3 supported above the wellbore 1. As is known, derricks, slips and other similar systems may be used in lieu of a surface truck for inserting and retrieving the perforating system into and from a wellbore. Moreover, perforating systems are also disposed into a wellbore via tubing, drill pipe, slick line, and/or coiled tubing.
Included with the perforating gun 6 are shaped charges 8 that typically include a housing, a liner, and a quantity of high explosive inserted between the liner and the housing. When the high explosive is detonated, the force of the detonation collapses the liner and ejects it from one end of the charge 8 at very high velocity in a pattern called a “jet” 12. The jet 12 perforates the casing and the cement and creates a perforation 10 that extends into the surrounding formation 2.
FIG. 2 is a prior art example of a portion of a perforating string 4 showing a connection between a connector sub 13 and perforating gun 6. A typical way of transferring a detonation wave between adjacent members of a perforating string 4 involves providing a detonating cord 15, and attaching a transfer charge 17 on its lower end that is oriented towards the perforating gun 6. A solid bulkhead 19 may sometimes be provided in the connection between adjacent members of a perforating string 4. To transfer the detonation wave into the perforating gun 6, a booster charge 21 is often set within the perforating gun 6 that faces the transfer charge 17. Per design, a jet (not shown) forms upon detonation of the transfer charge 17 that penetrates the bulkhead 19 to detonate the booster charge 21. This perpetuates travel of the detonation wave from the perforating cord 15 and the connecting sub 13 to the perforating cord 15 within the perforating gun 6. Shaped charges 8 are generally positioned adjacent the perforating cord 15, each having high explosive that explodes in response to the detonation wave traveling along the perforating cord 15.
As shown in a side sectional view in FIG. 3, in some instances adjacent perforating members 6A, 13A in a perforating string 4A may pivot with respect to one another and be oriented at an angle oblique to one another. This is sometimes accomplished by providing a spherically-shaped ball end 23, which is shown on the upper end of the perforating gun 6A, and mating the ball end 23 with a socket end 25, shown on the lower end of the connector sub 13A. The socket end 25, which is also spherically-shaped to accommodate the profile of the ball end 23, allows a pivoting action of the two perforating string members 6A, 13A. Problems associated with current transfer mechanisms include the bulkhead 19 blocking or substantially hindering the jet from the transfer charge 17 so that an insufficient amount of energy in the jet to detonate the booster charge 21. Such an instance halts transfer of the detonation wave into any perforating cord(s) 15 lower in the perforating string 4A. The swiveling connections of FIG. 2 also pose a problem in that the pivoting sometimes moves the booster charge 21 out of the path of the jet formed by the transfer charge 17, which also prevents further travel of the detonation wave within the perforating string.